Re: functional dependencies

Dear Andre, Larry and Guillermo,

      I am sorry in the posting I have made just some time ago I acknowledged that Andre raised the point that the defn. does not say that the tuples be distinct when it was Guillermo. I am sorry for that. Since Andre had written the mail aliased as Tony that led to the whole confusion and I am sorry for that. Regards,
Rohan.

Guillermo Labatte <labatteg_at_yahoo.com> wrote in message news:<pan.2003.06.12.11.38.04.386977.6374_at_yahoo.com>...
> On Thu, 12 Jun 2003 03:48:26 -0300, Rohan Hathiwala wrote:
>
> > Dear Andre and Larry,
> > Larry, let me quote a few lines from your recent posting.
> >
> > "Yes, the primary key, by definition functionally determines
> > all non-key attributes in the tuple. The same is true for any
> > candidate key, not just the primary key."
> >
> > That is precisely what I am having trouble with. As I see it both
> > functional dependencies and primary keys are seperate integrity
> > constraints.
>
> They are not. Primary (or candidate) keys are just special cases of FDs.
>
> > So if in a FD say A -> B if we make A a primary key the
> > whole thing degenerates to a primary key constraint and according to me
> > it does not remain a FD anymore.
>
> But not according to everyone else.
>
> > because by definition of FD we can have
> > two tuples t1 and t2 with t1[A] = t2[A] but if A is primary key this
> > will never happen. To make it more clear I have trouble accepting that a
> > primary key functionally determines all non-key attributes.
>
> Lets revisit the definition you gave
>
> "In a relation R if we have 2 attributes A and B and suppose an
> FD exists i.e A -> B then for any two tuples t1 and t2 in
> R if t1[A] = t2[A] the we must have t1[B] = t2[B]"
>
> The definition says "for any two tuples t1 and t2 in R", but it does not
> require that these two tuples be distinct, so the definition includes
> those cases in which t1 = t2. So if A is primary key, you will find that
> t1[A] = t2[A] is true when t1 = t2.
Received on Fri Jun 13 2003 - 08:19:50 CEST